New Charging Technique Puts Crumbling Batteries Back Together

For electric cars to run as long as possible between charges, their batteries need to pack a punch. One option would be lithium-metal batteries, which have a key component made of this energy-dense element. This gives them greater storage capacity than widely used lithium-ion batteries, in which the same component is made from graphite. Although lithium metal can store more energy than graphite, it also degrades faster, limiting how many times a lithium-metal battery can charge and discharge. But researchers have found that a new charging technique can actually restore the damaged material, extending this battery’s lifetime by about 30 percent.

As any rechargeable battery charges and discharges, lithium ions move back and forth between the positively-charged cathode and the negatively-charged anode. When this anode is also made of lithium, as it is in lithium-metal batteries, this process gradually causes small pieces of the reactive material to break off from the anode’s body. Within the battery, the lost chunks form tiny lithium “islands” that most researchers had considered inactive—until now. Stanford University researchers found that these isolated bits could still respond electrically, physically moving back and forth as the battery charged and discharged. Their discovery was published in Nature.

The researchers found that the islands could wiggle around enough to reestablish an electrical connection between the isolated lithium and the anode. They realized they could coax the material back together by immediately discharging a small amount of electricity after the battery had charged to capacity. “That’s how we promote [the lost lithium’s] growth toward the anode to reestablish the electrical connection,” says the study’s lead author and Stanford materials scientist Fang Liu. When they charged a lithium-metal test battery using this protocol, it could perform more charging cycles, lasting 29 percent longer than a battery that underwent standard charging.

Kelsey Hatzell, a Princeton University electrochemical and materials scientist who was not involved in the study, says the finding contributes to the fundamental understanding of lithium-metal batteries. “Observing … the dynamics of isolated lithium metal is very challenging,” she says, adding that the researchers “have designed a lot of very intriguing experiments to start to deconvolute the mechanisms.” She notes, however, that practical applications may be far off; these batteries still fall short of the thousands of charging cycles that rechargeable batteries must endure.

The Stanford researchers hope to further develop their charging method to maximize lithium-metal battery lifetime. They are also working on a charging protocol that would extend lithium-ion batteries’ usability. “I will consider [this study] as a major discovery for the battery field—lithium-ion, lithium-metal,” says co-author and Stanford materials scientist Yi Cui. “It can be generalized, I think, to the whole battery field.”

Reference-www.scientificamerican.com

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